Microalgae offer great promise to contribute a significant portion of the renewable fuels that will be required to meet the U.S. biofuel production target of 36 billion gallons by 2022, as mandated in the Energy Independence and Security Act of 2007 under the Renewable Fuels Standard. In the longer term, biofuels derived from algae represent an opportunity to dramatically impact the U.S. energy supply for transportation fuels. The cultivation of algae at a commercial scale could provide sufficient fuel feedstock to meet the transportation fuels needs of the entire United States, while being completely compatible with the existing transportation fuel infrastructure. Further, algal biofuels could prove sustainable for generations—they consume CO2 as a nutrient, have a much higher yield potential than other terrestrial biomass feedstocks, and can be grown with non-fresh water sources without needing to use high-value arable land. However, despite their huge potential, the state of technology for producing algal biofuels is regarded by many in the field to be in its infancy. There is a general consensus that a considerable amount of research needs to be carried out to produce algal-based fuels sustainably and economically enough to be cost-competitive with petroleum-based fuels.
Currently, algae are commercially cultivated in open raceway ponds. Though algae are photosynthetically very efficient and yield 5-10 times more biomass productivity than terrestrial plants, harvesting of algae from the growth medium is still considered to be a significant challenge. In general the concentration of algae in the open ponds is about 0.1-0.5 g/L. Current harvesting costs using a continuous-flow centrifuge have been estimated to be about $1500-2000/ton of dry algal biomass. Any algae-to-fuel strategy, therefore, must consider the energy costs and issues associated with harvesting and dewatering.
Bivalve mollusks are filter-feeding and naturally consume algae as a “bioconverter”. These bivalve mollusks feed on the algae biomass and convert algal lipids into molluscan biomass, or discharge the concentrated algal cells as their pseudofeces. Apart from easy harvesting by filtration, pseudofeces have much lower water content than the algae and a more desirable biochemical makeup for lipid extraction (Iritani et al., (1980) J. Nutrition 110: 1664-1670). Both the pseudofeces and clam biomass, after removal of shells, can be converted into biocrude or biofuel through advanced thermochemical liquefaction technique without drying the biomass.
Corbicula mussels were introduced to North America 50 years ago and are now ubiquitous in rivers and lakes south of 40° latitude (Lauritsen, D. D. (1986) J. N. Am. Benthol. Soc. 5: 165-172). They have a short lifespan, high fecundity and fast growth rate (McMahon, R. F. (1982) Nautilius 96: 134-141; Ortmann & Grieshaber (2003) J. Exp. Biol. 206: 4167-4178). They thrive successfully in systems receiving agricultural and industrial effluent, pollutants, and urban waste (Graczyk et al., (1997) Parasitol, Today 13: 348-351). Up to 3,750 individuals per m2 have been recorded in high-nutrient agricultural drainages (McMahon, R. F. (1991) In Thorp & Covich (Eds) Ecology and Classification of North American Freshwater Invertebrates Acad. Press, pp 315-399) and densities of 100 to 350 clams/m2 are common in Southeastern streams and rivers (Laurisen & Mozley (1986) Water Resources Res. Inst. Report #192, U. N. C.; McMahon, R. F. (1983) In Russell-Hunter W. D. (ed) The Molllusca, Acad Press). Native mollusks (Unionidae and Pisidiidae) are less abundant, have lower filtration rates than C. fluminea, and typically do not tolerate low oxygen-high nutrient environments (Mattice 1979). C. fluminea are preferential filter feeders rather than feeding on detritus (McMahon 1991). These mussels can filter a large range of particle sizes (5-30,000 μm) and are not adversely affected by filtering and feeding on cyanobacteria (Lauritsen, D. D. (1986) J. N. Am. Benthol. Soc. 5: 165-172; Wallace et al., (1977) Arch. fur Hydrobiologie 79: 506-532), many of which can produce toxins (Carmichael et al., (1992) J. Appl. Bact. 72: 445-459). A dense bed of C. fluminea filtered the overlying water column (average depth=5.25 m) of a North Carolina river in approximately 1-1.6 days (Lauritsen, D. D. (1986) J. N. Am. Benthol. Soc. 5: 165-172) or 1-2 L/hour/individual (Haven & Morales-Alamo (1970) Biol. Bull. 139; 248-264; Hildreth & Crisp (1976) J. marine Biol. Assoc. U.K. 56: 111-120; Winter, J. E. (1970) In Steele J. H. (ed.) Marine Food Chains Oliver & Boyd pp. 196-206).